A modified energy-based low cycle fatigue model for eutectic solder alloy

Abstract

Gintic Institute of Manufacturing Technology, Nanyang Drive, Singapore 638075*School of Mechanical and Production Engineering, Nanyang Technological University,Nanyang Avenue, Singapore 639798(Received April 21, 1999)(Accepted May 7, 1999)1. IntroductionSurface mount technology (SMT) is increasingly used in microelectronics to mount components bysoldering onto the printed circuit board (PCB). The solder alloys are used as the electrical andmechanical connections between the component and the board. Fatigue failure of solder joints isrecognized as a major cause of failure in electronic devices. An approach to this problem is to determinethe fatigue behaviors of solder alloy by accelerated fatigue testing at different temperatures. In thepresent research, smooth specimens made entirely of a 63Sn/37Pb solder alloy were tested over a widetemperature range at various low frequencies to study its low cycle fatigue properties.Strain-based models, notably the Coffin-Manson model (1–2), have been widely used to characterizelow cycle fatigue behaviors of engineering materials including solder alloys (3–5). However, it ispractically very difficult to obtain a single plastic strain value in a solder joint because of the complexstress state (6). In contrast, it is much easier to calculate energy density from the low cycle hysteresisloops for any types of solder joints under test (7). Therefore, in recent years, energy-based low cyclefatigue models have been increasingly used for solder alloys (7–9). These models are not so wellestablished as the strain-based ones, and some researchers (10) even expressed doubt on whether energydensity is a true parameter governing fatigue life of solder alloy. Therefore, in the study special effortwas made to examine the existing energy-based models. In the end, a flow stress modified energy-basedmodel is proposed based upon the examination and the experimental results. It is demonstrated in thepaper that the new fatigue model can be used to predict the fatigue life of solder alloy at differentfrequencies or temperatures.2. Experimental ProceduresThe material used in the study was a eutectic alloy 63Sn/37Pb. This material is widely used as a solderin SMT. The chemical composition of the solder is as follows (in wt.%): 63.2 Sn, 0.006 Sb, 0.002 Cu,0.004 Bi, 0.001 Zn, 0.002 Fe, 0.001 Al, 0.01 As, 0.001 Cd, and remainder Pb. Rods of the solder wereproduced by casting and then machined into cylindrical fatigue specimens of 90 mm long. The fatiguespecimens have a diameter of 12 mm at the two ends and a central diameter of 6 mm with a radius ofcurvature of 105 mm in the gauge section to prevent any stress concentration due to sharp corners. After